Question

Conc.${{\text{H}}_2}{\text{S}}{{\text{O}}_4}$ in addition to dry ${\text{KN}}{{\text{O}}_3}$, gives brown fumes of:
A. ${\text{S}}{{\text{O}}_{\text{2}}}$
B. ${\text{S}}{{\text{O}}_{\text{3}}}$
C. ${\text{NO}}$
D. ${\text{N}}{{\text{O}}_2}$

Answer 1

Hint: When concentrated sulphuric acid is added to the dry potassium nitrate, the concentrated sulphuric acid reacts with dry potassium nitrate. By the reaction gas is formed. The colour of the gas is brown.

Complete step by step answer:
When concentrated sulphuric acid is added to the dry potassium nitrate, the concentrated sulphuric acid reacts with dry potassium nitrate, so the brown fumes are observed.
The reaction of concentrated sulphuric acid and dry potassium nitrate is as follows:
Concentrated sulphuric acid reacts with dry potassium nitrate to form nitric acid and potassium sulphate.
${{\text{H}}_2}{\text{S}}{{\text{O}}_4}\,{\text{ + }}\,{\text{KN}}{{\text{O}}_3}\, \to \,{\text{HN}}{{\text{O}}_{\text{3}}}\, + \,\,{\text{KHS}}{{\text{O}}_{\text{4}}}$
The formed nitric acid dissociates to give nitrogen dioxide, water and oxygen gas.
The equation of dissociation of nitric acid is as follows:
$4\,{\text{HN}}{{\text{O}}_{\text{3}}}\,\, \to \,{{\text{O}}_{\text{2}}}\,{\text{ + 2}}\,\,{{\text{H}}_{\text{2}}}{\text{O}}\,{\text{ + }}\,\,4\,{\text{N}}{{\text{O}}_2}$
The gas formed by the reaction of concentrated sulphuric acid and dry potassium nitrate is nitrogen dioxide. The colour of nitrogen dioxide gas is brown. The gas releases from the reaction mixture so the brown fumes are observed.

Therefore, option (D) ${\text{N}}{{\text{O}}_2}$ is correct.

Note:
Different gases show a different colour. The colour is used for the identification of gas. Oxygen, sulphur dioxide, sulphur trioxide and nitrogen oxide are colourless gases. Nitrogen dioxide is an acidic gas. The nature of gas is determined by dissolving the gas in water. The gas which produces acid is known as acidic gas. The nitrogen dioxide produces nitric acid on dissolving in water.